Pokemon.com has announced the first post-release WiFi event for Pokemon HeartGold and Soulsilver.
From April 1st through May 5th you can download a Pokewalker route called the Yellow Forest.
This new Pokewalker route features, as the name implies, Pikachu. There are even some special variations of Pikachu you can catch when walking this route.
This is NOT an in-store event, this is only available through direct Nintendo WiFi, which means you have to have configured your DS to connect to the internet via WiFi.
Details on how to do that are in the manual that came with your game. (Page 35 in the SoulSilver manual.)
Once you have that setup, step by step download instructions are available at the Pokemon HeartGold and SoulSilver web site.
[Edit: This event is now over. The next event has been announced.]